transpiration
Why do plants need transport
Some small or primitive plants, such as mosses, absorb all the nutrients they need
directly from their environment.systems?
Larger plants do not have a large enough
surface area to take in what they need. Like
most multicellular animals, they have
developed specialized tissues for
transporting water and nutrients to all their
cells.
Plants that have specialized transport systems
are known as vascular plants.
Transporting water and nutrients
Plant transport tissues
Plant anatomy
What is transpiration?
Transpiration is the loss of water from the leaves of a plant. Most of this occurs from the
underside of a leaf, where there are many stomata in the epidermis.
Most plants control their water intake by
opening and closing their stomata. This
happens when water levels change in the
guard cells around each stoma. This occurs
either passively by osmosis, or by active
transport of solutes.
Transpiration rates also vary naturally in response to environmental factors such as
temperature and humidity.
What is water potential?
Water tends to move from areas of high water concentration to areas of low water
concentration. This is osmosis.
Water also tends to move from areas of
high hydrostatic pressure to areas of low
hydrostatic pressure. It is also affected by
gravity and electrostatic forces, such as
those that cause surface tension.
The collective term for the tendency of water to move due to any of these effects is
water potential.
Cohesion–tension theory
Water is a polar molecule, meaning that its positive and
negative charges are not evenly distributed. The oxygen atom
has a slight negative charge, while the two hydrogen atoms
are slightly positive.
This means that, in the xylem, water molecules
spontaneously arrange so that positive and negatively
charged poles lie next to each other.
This causes the molecules to cohere, or stick together, so
that as some leave a plant by transpiration, others are pulled
up behind them.